Use of electrically conductive oil-in-water microemulsions based on perfluorinated compounds as catholytes in electrochemical processes

Abstract

 An electrochemical process is described wherein a gaseous sub­stance is reduced at the cathode and in which microemulsions of the oil-in-water (o/w) type are utilized as catholytes, said microemulsions showing electric ion transfer and inter­phase matter transfer capacity. The oil phase of said micro­emulsion consists of perfluoropolyethers having perfluoroalkyl end groups or hydrophilic functional end groups or of perfluo­rocarbons and said microemulsions are obtained by using per­fluorinated surfactants, in particular those having a perfluo­roalkylpolyether structure,and/or by using an alcohol as co­surfactant.

Description

[0001] The present invention relates to the use of perfluorinated oil-in-water (o/w) microemulsions as catholytes in electroly­tic processes. In particular, the perfluorinated oils are preferably of the perfluoropolyether type.

[0002] There is a need of having available electrochemical processes in which it is possible to obtain a high current density with the minimum cell voltage. Said need can be satisfied, for ex­ample, by reducing the hydrogen discharge overvoltage with the aid of catalyzing electrodes as cathodes.

[0003] A possible alternative thereto is a cathodic reaction which - with the anodic reaction being the same - occurs at a lower value of the reversible potential difference.

[0004] It is well known from electrochemical processes, in particu­lar from voltametry, that a gas-saturated (for example O₂-­saturated) saline solution exhibits certain limiting values of the reduction current of said gas as a function of the tem­perature and of the angular revolving speed (ω) of the working electrode, which are determined by the (low) solubility of the gas in the electrolyte. Conversely, the H₂ evolution cur­rent is only a function of the potential and of the tempera­ture, since the reduction of H⁺ ions to H₂ is substantially independent of diffusion and, therefore, independent of ω.

[0005] It has now surprisingly been found that by carring out vol­tametric processes in the o/w microemulsions described herein­below, having an electric conductance of preferably at least 1 milliS.cm⁻¹, the diffusion limiting current density of the gas which is being reduced at the cathode is much higher than the diffusion limiting current density of the same gas in an aqueous (saline) solution, when working under identical con­ditions as to temperature and rotational speed of the elec­trode.

[0006] A further surprising aspect of the present invention is that - with the current density and the anodic process being the same - the difference between the cathodic potential of a pro­cess in a microemulsion (for example the reduction of O₂ to OH⁻) and the cathodic potential of a reference process in an aqueous solution (typically the H₂ evolution) is such that as compared to the electrolysis in aqueous phase the electro­lysis in microemulsion allows to save energy.

[0007] In certain cases it is possible to observe in the microemul­sion a considerable current, due to the reduction of oxygen, at cathode potentials at which no discharge of H⁺ can be ob­served in the reference solution.

[0008] It is apparent that it is necessary to compare - under the same current conditions - the cathodic process of O₂ reduction in microemulsion with the H₂ evolution in aqueous solution, since reduction of O₂ in aqueous solutions can only occur at a low current density, limited by the low solubility of the gas and,consequently,by the diffusion process.

[0009] Thus, object of the present invention is an electrochemical process wherein a gaseous matter is reduced at the cathode and wherein oil-in-water (o/w) microemulsions having an elec­tric conductance (due to ion transfer) of at least 1 milli­siemens.cm⁻¹ are utilized as catholytes.

[0010] Preferably, microemulsions of perfluoropolyethers and/or per­fluorocarbons in water having an electric conductance of at least 1 millisiemens.cm⁻¹ are used as catholytes for the ca­thodic reduction of oxygen.

[0011] Microemulsions employable according to the present invention are described in IT-A-20,910 A/86, 19,494 A/87, 19,495 A/87, as well as in the corresponding EP-A-250 766 and 280 312, the contents of which are incorporated herein.

[0012] Whenever used in the present invention, the term microemulsion also includes systems in which the molecular orientation in the interphase leads to the formation of optically an-isotro­pic systems, characterized by bi-refraction and probably con­sisting of oriented structures of the liquid crystal type.

[0013] The microemulsions used according to the present invention are mixtures which macroscopically consist of only one limpid or opalescent phase, which is indefinitely stable in the ope­rative temperature range. Said mixtures preferably comprise:

a) an aqueous liquid optionally containing one or more electro­lytes (e.g. salts like alkali and alkaline earth metal ha­lides, nitrates, sulfates, phosphates, etc. and the corres­ponding acids);

b) a fluid containing one or more species of perfluoropolyether structure having perfluoroalkyl and/or functional end groups, with one or more functionalities selected from carb­oxyl, alcohol, polyoxyalkylene-OH, ester, amide, etc., and preferably hydrophilic functional groups, such as the carboxyl and polyoxyalkylene-OH groups, and particularly carboxylic groups;

c) a fluorinated surfactant preferably comprising (or consist­ing of) one or more species of perfluoropolyether structure; and/or

d) a hydrogenated (non-fluorinated) C₁₋₁₂, preferably C₁₋₆ alcohol and, optionally, a fluorinated (preferably C₁₋₁₂) alcohol (co-surfactant).

[0014] The microemulsions used herein may be optically isotropic or birefractive, are of the oil-in-water (o/w) type and are cha­racterized in that they are conductive, their conductance be­ing at least equal to 1 millisiemens.cm⁻¹.

[0015] Since the microemulsions used according to the present inven­tion are of the o/w type, they must contain the (preferred) PFPE or the perfluorocarbon as "dispersed phase". Therefore, the aqueous phase should preferably be in excess (as to the volume) with respect to the perfluoropolyether (or perfluoro­carbon) phase.

[0016] Perfluoropolyethers (PFPE) suitable for forming the microemul­sions employed according to the present invention are:

a) PFPE having an average molecular weight ranging from 500 to 10,000, preferably from 600 to 6,000, having perfluoro­alkyl end groups and belonging to one or more of the fol­lowing classes:

1)

wherein the perfluorooxyalkylene units are distributed statistically and R_f and R′_f, the same or different from each other, are -CF₃, C₂F₅ or -C₃F₇, and m, n, p have such average values as to meet the above requirement as to average molecular weight;

2)R_fO(CF₂CF₂O)_n(CF₂O)_mR′_f, wherein the perfluorooxyalky­lene units are statistically distributed, R_f and R′_f, the same or different from each other, are -CF₃ or -C₂F₅, and m and n have such average values as to meet the above requirement as to average molecular weight;

3)

wherein the perfluorooxyalkylene units are statistically distributed, R_f and R′_f, the same or different from each other, are -CF₃, -C₂F₅ or -C₃F₇, and m, n, p, q have such average values as to meet the above requirements as to average molecular weight;

4)

wherein R_f and R′_f, the same or different from each other, are-C₂F₅ or -C₃F₇ and n has such an average value as to meet the above requirement as to average molecu­lar weight;

5) R_fO(CF₂CF₂O)_nR′_f, wherein R_f and R′_f, the same or dif­ferent from each other, are -CF₃ or -C₂F₅ and n has such an average value as to meet the above requirement as to average molecular weight;

6) R_fO(CF₂CF₂CF₂O)_nR′_f, wherein R_f and R′_f, the same or dif­ferent from each other, are -CF₃, -C₂F₅ or -C₃F₇ and n has such an average value as to meet the above require­ment as to average molecular weight;

7) PFPE having the structure of class 1 or 3, in which one of the two end groups R_f or R′_f contains one or two chlorine atoms, such PFPE being described in e.g. IT-­A-20,406 A/88 of the applicant.

b) PFPE belonging to the above classes 1) to 7), having an average molecular weight ranging from 1,500 to 10,000, and preferably lower than 6,000, but containing on the average from 0.1 to 4, (e.g. 0.1 to 2) and preferably from 0.3 to 1, non-perfluoroalkyl end groups per polymer chain;

c) Perfluoropolyethers as e.g. described in IT-A-20,346 A/86 in the name of the applicant, having functional groups along the perfluoropolyether chain and end groups of the perfluo­roalkyl or functional type.

[0017] As examples of non-perfluoroalkyl end groups and of functional groups in the chain there may be mentioned, for example, carb­oxylate, alcohol, polyoxyalkylene-OH, etc. groups.

[0018] The most suitable functional end groups and functional groups in the chain are the hydrophilic ones,and in particular carb­oxyl groups.

[0019] The functional end groups as well as the functional groups in the chain mentioned above can be linked to the perfluoropo­lyether chain through a -CFX-group in which X is F or CF₃, optionally followed by a linking group consisting of a divalent non-fluori­ nated radical of the alkylene or arylene type, containing up to 20 carbon atoms, preferably containing 1 to 8 carbon atoms, according to the sequence: perfluoropolyether chain -CFX- non-­fluorinated radical-functional group. Specific examples of suitable divalent non-fluorinated radicals are: methylene, ethylene, propylene, butylene, pentylene, hexylene, octylene, phenylene, biphenylene and naphthylene.

[0020] It is to be understood that perfluoropolyethers also employ­able according to the present invention are the ones of clas­ses 1, 2 and 3, which contain peroxy bridges in the chain and have acid end groups, said PFPE being obtainable as crude pro­ducts of the photo-oxidation process utilized for the synthe­sis of the above PFPE.

[0021] Perfluoropolyethers of class 1) are commercially available under the trade marks Fomblin® Y or Galden®; the ones of class 2) are marketed under the trade mark Fomblin® Z. All of said products are produced by Montedison S.p.A.

[0022] Commercially known products of class 4) are the Krytox® (Du Pont) PFPE. The products of class 5) are described in US-A-­4,523,039 whereas the products of class 6) are known from EP-­A-148,482.

[0023] The products of class 3) may be prepared according to US-A-­3,665,041.
Other suitable perfluoropolyethers are the ones described by Lagow et al. in US-A-4,523,039 or in J. Am. Chem. Soc. 1985, 7, 1197-1201.

[0024] The fluorinated surfactants contained in the present micro­emulsions may be ionic or non-ionic. Examples of preferred surfactants are:

a) the salts of perfluoroalkylcarboxylic acids having 5 to 11 carbon atoms;

b) the salts of perfluorosulphonic acids having 5 to 11 carbon atoms;

c) the non-ionic surfactants described in EP-A-51526, consi­sting of a perfluoroalkylene chain and a hydrophilic poly­oxyalkylene cap (head);

d) the salts of mono- and di-carboxylic acids derived from perfluoropolyethers;

e) the non-ionic surfactants consisting of a perfluoropolyether chain linked to a polyoxyalkylene chain;

f) the perfluorinated cationic surfactants or the surfactants derived from perfluoropolyethers having 1, 2 or 3 hydropho­bic chains.

[0025] The preferred surfactants are those of the ionic type. Pre­ferred cations in the salts mentioned above are NH₄⁺ and the cations of alkali and alkaline earth metals, e.g. Na, K, Mg and Ca.

[0026] Furthermore, the system may contain one or more surfactants belonging to one of the following classes:
- hydrogenated alcohols having 1 to 12 carbon atoms (e.g. me­thanol, ethanol, propanol, butanol, pentanol, hexanol, octa­nol and decanol);
- alcohols comprising a perfluoropolyether chain,
- partially fluorinated alcohols.

[0027] The aqueous liquid may consist of water or of an aqueous solu­tion of inorganic electrolytes (salts, acids or alkalis).

[0028] The present o/w microemulsions which are utilizable as catho­lytes for cathodic gas reduction reactions may also comprise, as dispersed oil phase, a perfluorocarbon in addition to or (preferably) instead of a perfluoropolyether, provided that such a microemulsion has a conductance of at least 1 milli­siemens.cm⁻¹.

[0029] Perfluorocarbon microemulsions are well known in the art - see for example EP-A-51,526, the contents of which are in­corporated herein. Perferred perfluorocarbons for use in the present invention contain from 5 to 15, particularly from 6 to 12, carbon atoms.

[0030] However, the use of conductive o/w emulsions, in which the oil is a perfluoropolyether, is particularly preferred.

[0031] The microemulsions to be used as catholytes may be prepared by mixing the individual components taken in any order.

[0032] According to the present invention there are used,as catho­lytes, o/w microemulsions having a conductivity of at least 1 millisiemens.cm⁻¹ with respect to the electrolytic reac­tions of any gas that can be reduced at the cathode. In the following examples oxygen has been used as gas and therefore all the voltametric tests reported hereinafter and the corres­ponding evaluations concern the cathodic reaction:
O₂+2H₂O+4e⁻ → 4OH⁻,
but it is apparent that said evaluations are to be considered as only illustrative.

[0033] From the voltametry in aqueous (NH₄)₂SO₄ solution, saturated with O₂, there were obtained the values of the diffusion li­miting current of O₂-reduction as a function of temperature and angular rotational speed (ω) of the working Pt electrode, as well as the values of the H₂ evolution current as a func­tion of potential and temperature, since the H⁺ reduction is substantially independent of the diffusion and therefore is independent of ω.

[0034] Voltametric measurements were carried out by using, as catho­lyte, the microemulsion (ME). at the same temperatures and at the same ω, determining:

1) O₂-diffusion limiting current density and increase thereof with respect to the diffusion current of the same in an aqueous medium;

2) cathodic potential difference - with the current density being the same - between a cathodic process in microemul­sion (typically O₂ reduction) and a reference cathodic pro­cess in aqueous solution (typically H₂ evolution).

[0035] The limiting current of O₂-reduction indicated in each example is always referred to a cathodic potential which is lower (by 200 mV) than the value at which H₂-evolution in the examined system starts.

[0036] In order to measure the current as a function of the applied potential a voltametry was conducted, using as catholytes va­rious ME and as anolyte a concentrated aqueous solution of an inorganic electrolyte.

[0037] Voltametric measurements were carried out by means of a multi­polarograph Amel® 472, in a 3-electrode cell:
- working electrode of the Pt rotating disc type, having a geometric surface area of 3.14 mm², immersed in the ME;
- Pt counter-electrode, immersed in an aqueous solution of (NH₄)₂SO₄ (3 moles/liter), separated from the ME by an agar-­agar septum;
- reference calomel electrode (SCE) immersed in a saline bridge (KCl solution, 3 moles/l) with a Luggin capillary facing the working electrode surface.

[0038] All the cathodic potential values reported hereinbelow are referred to the SCE (standard calomel electrode).
As working electrodes it is possible to use all of those which are generally utilized for the cathodic gas reduction, for example those made of Pt, Au or Ni. Platinum is particularly preferred.

[0039] In each test, about 60 ml of ME, at the desired temperature, were saturated with moist O₂ at atmospheric pressure.

[0040] Starting from the spontaneous potential of the system in the absence of current, a potential sweep-100 mV s⁻¹ was applied to the working cathode, and the circulating current was recor­ded as a function of the cathodic potential for different ro­tational speeds of the electrode.

[0041] In a concentrated aqueous solution of (NH₄)₂SO₄ (3 moles/l, corresponding to 396 g/l) at a pH of 5.3 and at a specific con­ductance of 172 milliS.cm⁻¹, H₂-evolution occurred at a catho­dic potential of higher than -700 mV (SCE).

[0042] In this case the limiting current of O₂-reduction observed at 20°C was 2-3 µ A mm⁻² in the absence of stirring, and was 5 µ A mm⁻² with ω = 1,500 rpm; at 40°C, 3 µ A mm⁻² were ob­tained with ω = 0, and about 10 µ A mm⁻² were obtained with ω = 1,500 rpm.

[0043] At 60°C and without electrode rotation, the observed limiting current density was 30 µ A mm⁻².
As regards the comparison of the potentials at which the same cathodic current density is observed both in microemul­sion and in electrolytic aqueous solution, said comparison was conducted at the same temperature and at a pH of the aque­ous electrolyte solution as close as possible to the pH of an aqueous solution of the fluorinated surfactant utilized for preparing the microemulsion.

[0044] The following example is to be considered as merely illustra­tive but not limitative of the present invention.

EXAMPLE 1

[0045] The microemulsion sample was prepared by mixing 3.24 g of mo­ nocarboxylic acid having a perfluoropolyether structure and an average equivalent weight of 542, and 0.72 g of monocarb­oxylic acid having the same structure and an equivalent weight of 567, salified with 2 ml of an ammonia solution (10% by weight of NH₃), 7.20 g of perfluoropolyether having perfluoroalkyl end groups, belonging to class 1 and having an average mole­cular weight of 800, 1.44 g of an alcohol having a perfluoro­polyether structure and an average molecular weight of 678, 75 ml of water and 0.2 ml of an aqueous solution of KNO₃ (0.1 moles/liter).

[0046] The ME thus obtained was slightly opalescent at room tempera­ture and became fully limpid when heated to a temperature higher than 40°C.
It exhibited a conductance of 4.05 millisiemens.cm⁻¹, a neut­ral pH,and contained 14% by weight of substances with perfluo­ropolyether structure dispersed in H₂O.
From the voltametric diagrams obtained with this o/w ME, the following was determined:
- at 20°C, a limiting current density of O₂-reduction of 20 µA mm⁻² without stirring and equal to 55 µA mm⁻² at ω= 1500 rpm, both values being about 10 times higher than the ones obtained in an aqueous electrolyte solution;
- at 40°C, 30 µA mm⁻² circulated in the absence of stirring and about 70 µA mm⁻² at ω= 1500 rpm, these values being 10 and 7 times, respectively higher than the corresponding values measured in an aqueous solution.

[0047] To obtain a circulation of 50 µA mm⁻² at ω = 1500 rpm it was necessary to use, at 20°C, -580 mV instead of -750 mV required in an aqueous electrolyte solution under the same conditions, with a corresponding saving of energy of slightly less than 0.01 W/mm².

Claims

1. Electrochemical process wherein a gaseous substance is re­duced at a cathode and wherein microemulsions of the oil-­in-water (o/w) type are utilized as catholytes, said mic­roemulsions having an electric conductance by ion transfer of at least 1 millisiemens.cm⁻¹.

2. Process according to claim 1, wherein the gaseous matter is oxygen.

3. Process according to any one of claims 1 and 2, wherein the cathode is made of a metal generally utilized in vol­tametric processes.

4. Process according to claim 3, wherein the cathode is made of Au, Pt or Ni.

5. Process according to any one of claims 1 to 4, wherein the (o/w) microemulsions having a conductance of at least 1 millisiemens.cm⁻¹ are composed of a liquid, limpid or opa­lescent, macroscopically single-phased matter obtainable by mixing:

a) an aqueous liquid optionally containing one or more electrolytes;

b) a perfluoropolyether fluid having perfluoroalkyl and/or functional end groups, with carboxyl, alcohol, polyoxy­alkylene-OH, ester, amide etc. functionality, and pre­ferably functional groups of the hydrophilic type;

c) a fluorinated surfactant preferably having a perfluoro­polyether structure; and/or

d) a hydrogenated C₁₋₁₂ alcohol and, optionally, a (par­tially) fluorinated alcohol (co-surfactant).

6. Process according to claim 5, wherein the fluorinated sur­factant is selected from one or more of the following salts:

a) salts of perfluoroalkylcarboxylic acids having 5 to 11 carbon atoms;

b) salts of perfluorosulphonic acids having 5 to 11 carbon atoms;

c) salts of mono- and di-carboxylic acids derived from per­fluoropolyethers.

7. Process according to claim 5, wherein the fluorinated sur­factant is of the non-ionic type substituted by a perfluo­roalkyl chain and by a hydrophilic polyoxyalkylene cap.

8. Process according to any one of claims 1 to 7, wherein the oil is or comprises a perfluorocarbon.

9. Process according to any one of claims 1 to 7, wherein the oil is or comprises one or more perfluoropolyethers selec­ted from:

a) PFPE having an average molecular weight of from 500 to 10,000 and preferably from 600 to 6,000, with perfluoro­alkyl end groups, and belonging to one or more of the following classes:
1)

wherein the perfluorooxyalkylene units are distribu­ted statistically and R_f and R′_f, the same or diffe­rent form each other, are -CF₃, -C₂F₅ or -C₃F₇, and m, n, p have such average values as to meet the above requirement as to average molecular weight;
2) R_fO(CF₂CF₂O)_n(CF₂O)_mR′_f, wherein the perfluorooxy­alkylene units are statistically distributed, R_f and R′_f, the same or different from each other, are -CF₃ or -C₂F₅, and m and n have such average values as to meet the above requirements as to average molecu­lar weight;
3)

wherein the perfluorooxyalkylene units are statisti­cally distributed, R_f and R′_f, the same or different from each other, are -CF₃, -C₂F₅ or -C₃F₇, and m, n, p, q have such average values as to meet the above requirements as to average molecular weight;
4) wherein R_f and R′_f, the same or differnet from each other, are-C₂F₅ or -C₃F₇ and n has such an average value as to meet the above requirement as to average molecular weight;
5) R_fO(CF₂CF₂O)_nR′_f, wherein R_f and R′_f, the same or different from each other, are -CF₃ or -C₂F₅, and n has such an average value as to meet the above re­quirement as to average molecular weight;
6) R_fO(CF₂CF₂CF₂O)_nR′_f, wherein R_f and R′_f, the same or different from each other, are -CF₃, -C₂F₅ or -C₃F₇ and n has such an average value as to meet the above requirement as to average molecular weight;
7) PFPE having the structure of class 1 or of class 3, wherein one of the end groups R_f and R′_f, contains one or two chlorine atoms;

b) PFPE belonging to the above classes 1) to 7) and having an average molecular weight of from 1,500 to 10,000 but containing on the average from 0.1 to 4 non-perfluoro­alkyl end groups per polymer chain;

c) PFPE having functional groups along the perfluoropoly­ether chain and end groups of the perfluoroalkyl or functional type.